Transient permeation of organic vapors through polymer membranes

Abstract

A method of analyzing permeation data has been developed which permits the diffusion coefficient D, its variation with concentration, and the solubility coefficient to be determined from a single experiment on the permeation of a vapor through a membrane when diffusion is known to be Fickian. There is evidence that in the range from the glass temperature T_g to (T_g + 20°C.) the diffusion of an organic vapor into a polymer is not always Fickian. In such a case a single permeation experiment gives the limiting value of D at zero vapor concentration. If the sorption isotherm is determined, two or three permeation experiments give sufficient information to characterize the concentration dependence of D and to predict a “Fickian time lag.” The difference between the observed and the predicted time lag is the mean relaxation time of the time‐dependent factors in the non‐Fickian permeation. When there is a transient period of rapid permeation at the beginning of an experiment a plot of permeation rate against logarithmic time permits another relaxation time to be defined. By comparing these relaxation times with the mechanical characteristics of the polymer the time‐dependent mechanisms of permeation may be identified.